This invention relates to a multiple magnification mode copying apparatus, and particularly to such an apparatus which includes a full and half rate scanning mirror system, a lens for forming an image of an object to be copied, and means for shifting the lens along its optical axis to any one of a plurality of predetermined positions to change the magnification mode of the apparatus.
Typical of such an apparatus is a xerographic copying machine using a full and half rate scanning mirror system. In such a machine, if a standard lens is used, as well as shifting the lens along its optical axis, to change the magnification mode, the half rate mirror system is also moved to maintain the correct conjugate distances. The various necessary movements of the lens and half rate mirror system can be carried out in a relatively straight-forward manner if the datum line for documents to be copied, and the resultant copies, is taken to be the centre line of the system. In other words, documents and copies of different sizes need to be aligned so as to lie symetrically on either side of a centre line. This is known as a centre registration system. For the machine operator, however, the most convenient registration system is the edge registration system, where all documents and copies are made with one edge as the datum line. This gives rise to the need to move the lens transversely of the optical axis by the appropriate amount for each of the magnification modes. Furthermore, the relationships between the various necessary movements are complex, and if more than two magnification modes are required, the mechanisms for achieving the required positions of the lens and the half rate mirror system tend to be rather complicated.
If a zoom lens is used in a multiple magnification mode machine, although there is no need to shift the half rate mirror system for the different magnification modes, the relative positions of the lens elements must be changed. As with a copying apparatus using a standard lens, the zoom lens must also be shifted transversely of its optical axis in an edge registration system.
Examples of variable magnification copying machines using standard lens systems of the centre registration type, in which the lens and the half rate mirrors are shifted to vary the magnification, are described in U.K. patent specification No. 2074742A and in U.S. Pat. No. 4,168,905. Examples of variable magnification copying machines using zoom lens systems, and of the edge registration type in which the half rate mirrors are not shifted, but in which the lens is moved transversely of its optical access, are described in U.S. Pat. Nos. 2,059,083A and 2,073,899A.
An example of a dual magnification mode copying machine using a standard lens system, which is of the edge registration type, in which the lens is moved both along and transversely of its optical access, and in which the half rate mirror is shifted, is described in U.S. Pat. No. 3,614,222.